1. Sub-populations

2. Function

3. See also

4. References

5. Further reading

6. External links

Memory T cells are a subset of infection- and cancer-fighting T cells (also known as a T lymphocyte) that have previously encountered and responded to their cognate antigen; thus, the term antigen-experienced T cell is often applied. Such T cells can recognize foreign invaders, such as bacteria or viruses, as well as cancer cells. Memory T cells have become "experienced" by having encountered antigen during a prior infection, encounter with cancer, or previous vaccination. At a second encounter with the invader, memory T cells can reproduce to mount a faster and stronger immune response than the first time the immune system responded to the pathogen that entered the body. This behaviour is utilized in T lymphocyte proliferation assays, which can reveal exposure to specific antigens.

Sub-populations

Historically, memory T cells were thought to belong to either the effector or central memory subtypes, each with their own distinguishing set of cell surface markers (see below).^[1] Subsequently, numerous additional populations of memory T cells were discovered including tissue-resident memory T (Trm) cells, stem memory TSCM cells, and virtual memory T cells. The single unifying theme for all memory T cell subtypes is that they are long-lived and can quickly expand to large numbers of effector T cells upon re-exposure to their cognate antigen. By this mechanism they provide the immune system with "memory" against previously encountered pathogens. Memory T cells may be either CD4⁺ or CD8⁺ and usually express CD45RO.^[2]

Memory T cell subtypes:

• Central memory T cells (T_CM cells) express CD45RO, C-C chemokine receptor type 7 (CCR7), and L-selectin (CD62L). Central memory T cells also have intermediate to high expression of CD44. This memory subpopulation is commonly found in the lymph nodes and in the peripheral circulation. (Note- CD44 expression is usually used to distinguish murine naive from memory T cells).
• Effector memory T cells (T_EM cells) express CD45RO but lack expression of CCR7 and L-selectin. They also have intermediate to high expression of CD44. These memory T cells lack lymph node-homing receptors and are thus found in the peripheral circulation and tissues.^[3] T_EMRA stands for terminally differentiated effector memory cells re-expressing CD45RA, which is a marker usually found on naive T cells.^[4]
• Tissue resident memory T cells (T_RM) occupy tissues (skin, lung, gastrointestinal tract, etc.) without recirculating. One cell surface marker that has been associated with T_RM is the integrin αeβ7. These cells are thought to play a major role in protective immunity against pathogens.^[5][6] Dysfunctional TRM cells have been implicated in autoimmune diseases, such as psoriasis, rheumatoid arthritis, inflammatory bowel disease.^[6] Specific to TRMs are genes involved in lipid metabolism, being highly active, roughly 20- to 30-fold more active than in other types of T-cells.^[6]
• Virtual memory T cells (T_VM) differ from the other memory subsets in that they do not originate following a strong clonal expansion event. Thus, although this population as a whole is abundant within the peripheral circulation, individual virtual memory T cell clones reside at relatively low frequencies. One theory is that homeostatic proliferation gives rise to this T cell population. Although CD8 virtual memory T cells were the first to be described,^[7] it is now known that CD4 virtual memory cells also exist.^[8] Some have suggested that antigen-inexperienced memory T cells should be separated into ‘innate memory’ T cells and ‘virtual memory’ T cells.^[9]

There have been numerous other subpopulations of memory T cells suggested. For example, in the mouse, Sendai virus specific CD8+ T-cells low on CD43 expression mounted a higher memory recall response suggesting that memory CD8 T-cells can also be distinguished from activated effector CD8 T-cells using CD43 marker.^[10] Other investigators have studied Stem memory T_SCM cells. Like naive T cells, T_SCM cells are CD45RO−, CCR7+, CD45RA+, CD62L+ (L-selectin), CD27+, CD28+ and IL-7Rα+, but they also express large amounts of CD95, IL-2Rβ, CXCR3, and LFA-1, and show numerous functional attributes distinctive of memory cells.^[11]

Function

Antigen-specific memory T cells against viruses or other microbial molecules can be found in both T_CM and T_EM subsets. Although most information is currently based on observations in the cytotoxic T cells (CD8-positive) subset, similar populations appear to exist for both the helper T cells (CD4-positive) and the cytotoxic T cells.

• central memory (T_CM). The T_CM cells are thought to contain some properties associated with memory cells stem cells. T_CM display a capacity for self-renewal due to high levels of phosphorylation of an important transcription factor known as STAT5.^[12] In mice, T_CM cells have been shown to confer superior protection against viruses,^[13] bacteria,^[13] and cancer^[14] in several different model systems compared with T_EM cells.
• two highly related effector memory sub-types, which strongly express genes for molecules essential to the cytotoxic function of CD8 T cells:
  • effector memory (T_EM)
  • effector memory RA (T_EMRA)
• More recently, antigen-experienced CD8+ T cells with apparent self-renewal capabilities have been described in mice.^[15][16] This population, now termed stem cell memory (T_SCM), can be identified by the markers CD44(low)CD62L(high)CD122(high)sca-1(+) and are capable of generating T_CM and T_EM subsets while maintaining themselves. In preclinical studies, adoptively transferred T_SCM confer superior immunity compared with other T memory subsets.^[16] Whether such a population is found in humans is the subject of active investigation.
• After infection or inflammatory challenge, central memory CD8+ T cells rapidly traffic into nonlymphoid tissues, this cellular migration is driven by interleukin-15 stimulated enzymatic synthesis of core 2 O-glycans, which generates functional ligands for E- and P-selectins. In fact interleukin-15 stimulated expression of glycosyltransferase enzymes is largely a feature of TCM CD8+ T cells and this allows central memory T cell to selectively migrate out of the circulation and into nonlymphoid tissues. Therefore the entry of memory CD8+ T cells into inflamed, nonlymphoid tissues is primarily restricted to TCM cells that have the capacity to synthesize core 2 O-glycans.^[17]

See also

• Naive T cell
• V(D)J recombination of DNA
• Memory B cell

References

Further reading

• {{cite book |author=Janeway, Charles |title=Immunobiology: the immune system in health and disease |publisher=Garland Science |location=New York |year=2005 |pages= |isbn=978-0-443-07310-6}}
• {{cite book |author1=Lichtman, Andrew H. |author2=Abbas, Abul K. |title=Cellular and molecular immunology |publisher=Saunders |location=Philadelphia |year=2003 |pages= |isbn=978-0-7216-0008-6}}

External links

• T-cell Group - Cardiff University

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